FIR: use implicit Unit type in setters

compiler/testData/fir/rawBuilder/declarations/simpleClass.txt

@@ -17,7 +17,7 @@ FILE: simpleClass.kt
             public? get(): <implicit> {
                 STUB
             }
-            public? set(value: Boolean): <implicit> {
+            public? set(value: Boolean): kotlin.Unit {
             }
 
         public? final? lateinit property fau(var): Double

compiler/testData/fir/resolve/simpleClass.txt

@@ -17,7 +17,7 @@ FILE: simpleClass.kt
             public? get(): R|error: Not supported: FirImplicitTypeImpl| {
                 STUB
             }
-            public? set(value: R|kotlin/Boolean|): R|error: Not supported: FirImplicitTypeImpl| {
+            public? set(value: R|kotlin/Boolean|): R|kotlin/Unit| {
             }
 
         public? final? property fau(var): R|kotlin/Double|